Optimization of Lu-177 PSMA Post-Therapy Imaging Time

Purpose: Lu-177 PSMA is known as a theranostic agent due to therapeutic effects from beta minus decay and diagnostic imaging with gamma rays. In clinical practice, its imaging properties are underutilized with no standardized timing for imaging (except for dosimetry) and sometimes no imaging. At our clinic, scan times vary from one hour to 72 hours post-injection. This quality improvement project aims to compare scans performed at four and 24 hours to determine if there is a difference in image quality to guide treatment and incorporate the results into our clinic.

Methods/Materials: Our sample size included 88 Lu-177 PSMA therapy patients from November 2022-September 2023. Both whole body planar and targeted SPECT-CT images were obtained for each patient. Six nuclear medicine attending physicians were presented with a blinded selection of 10 patients who met these criteria and asked to assess image quality.

Results: Physicians accurately identified the timing of post-treatment scans 71% of the time (93% correct with 4-hour and 49% correct with 24-hour). Mann-Whitney U test showed a statistically significant higher image quality score for 24-hour scans (U = 241.5, p = 0.02). Physicians believed they could identify the same lesions on post therapy imaging as pre-treatment PET 66% of the time (50% with 4-hour imaging and 81% with 24-hour imaging) and that SPECT added value 67% of the time (localizing lesions, identifying smaller lesions).

Conclusions: Theranostics continue to expand as part of nuclear medicine, and thoughtful use of imaging is important to guide clinical decision making. We found that physicians prefer the image quality of 24-hour images but had difficulty differentiating them from 4-hour post-therapy scans. As our practice grows, further studies will help further elucidate the best method for post-treatment imaging.